For Nike(NKE) - Get Report investors, who have seen the sneaker king's shares tumble by about 13% this year, things may only get worse during the next six months.

Wall Street is growing concerned about the effect of rising competition on Nike from Under Armour(UA) - Get Report and Adidas. When Nike reports fiscal first quarter earnings on Tuesday after the close, the results may only stoke fear that Nike is losing ground to its arch rivals. Analysts estimate that first-quarter sales rose a meager 0.7% from the prior year to $8.88 billion. Earnings are seen plunging 16.4% year over year to 56 cents a share.

"Our concerns on Nike are mounting," Canaccord Genuity analyst Camilo Lyon said in a new note to clients ahead of the results. Here are two of the top concerns Lyon laid out that should have Nike's investors quaking in their Jordan high-tops.

Competition globally is heating up.

Nike's competitors have had the wind at their backs of late, and that is unlikely to change in the near-term. Adidas is seeing success with its classic Stan Smith sneakers and those backed by music star Kanye West as consumers shift to wearing more fashion-oriented athletic shoes, Lyon said. Meanwhile, Under Armour continues to expand its premium running footwear assortment at the same time it's chipping away at Nike's dominance in basketball thanks to Stephen Curry endorsed offerings which are cheaper than Nike's.

A prime example of the intensified competition weighing on Nike was explained rather simply by Finish Line's (FINL) CEO Sam Sato. "Nike's signature basketball [sales] were down in the [second] quarter," Sato told analysts on a Sept. 23 call. He added that Adidas' business "remains on fire" and that Under Armour saw "explosive" demand for the latest Curry basketball sneaker.

Nike feeling the footsteps of competitors isn't solely confined to the U.S. market.

"In addition to Under Armour's brand awareness sharply increasing in China via Stephen Curry, Adidas is also growing at an accelerated pace in China with sales up 30% in the most recent quarter excluding currency translation," Lyon said. Therefore, the Chinese market also looks as if it is becoming more competitive for Nike.

Under Armour could take business from Nike at Kohl's.

The red-hot athletic-wear maker will launch in about 600 of Kohl's(KSS) - Get Report 1,200 stores in the first quarter of 2017 in bid to reach more mainstream shoppers. Products will also be available online, and span the women's, men's, kid's, accessories, footwear and home departments.

"There is a customer walking into their stores [Kohl's] that just haven't been able to find our brand," said CEO Kevin Plank when announcing the distribution deal back in July.

Under Armour's arrival to Kohl's could come at the expense of long-time partner Nike. "Under Armour's entry into Kohl's next year could crimp $200 million to $300 million off of Nike's North American business as we
suspect orders that were mostly exclusive to Nike will get allocated to both brands," says Lyon.